Why Do We Sleep?

While we can more or less abstain from some basic biological
urges—for food, drink, and sex—we can’t do the
same for sleep. At some point, no matter how much espresso we
drink, we just crash. And every animal that’s been studied,
from the fruit fly to the frog, also exhibits some sort of
sleep-like behavior. (Paul Sternberg,
Morgan Professor of Biology, was one of the first to show that even
a millimeter-long worm called a nematode falls into some sort of
somnolent state.) But why do we—and the rest of the animal
kingdom—sleep in the first place?

“We spend so much of our time sleeping that
it must be doing something important,” says David Prober,
assistant professor of biology and an expert on how genes and
neurons regulate sleep. Yes, we snooze in order to rest and
recuperate, but what that means at the molecular, genetic, or even
cellular level remains a mystery. “Saying that we sleep
because we’re tired is like saying we eat because we’re
hungry,” Prober says. “That doesn’t explain why
it’s better to eat some foods rather than others and what
those different kinds of foods do for us.”

No one knows exactly why we slumber, Prober says, but there are
four main hypotheses. The first is that sleeping allows the body to
repair cells damaged by metabolic byproducts called free radicals.
The production of these highly reactive substances increases during
the day, when metabolism is faster. Indeed, scientists have found
that the expression of genes involved in fixing cells gets kicked
up a notch during sleep. This hypothesis is consistent with the
fact that smaller animals, which tend to have higher metabolic
rates (and therefore produce more free radicals), tend to sleep
more. For example, some mice sleep for 20 hours a day, while
giraffes and elephants only need two- to three-hour power naps.

Another idea is that sleep helps replenish fuel,
which is burned while awake. One possible fuel is ATP, the
all-purpose energy-carrying molecule, which creates an end product
called adenosine when burned. So when ATP is low, adenosine is
high, which tells the body that it’s time to sleep. While a
postdoc at Harvard, Prober helped lead some experiments in which
zebrafish were given drugs that prevented adenosine from latching
onto receptor molecules, causing the fish to sleep less. But when
given drugs with the opposite effect, they slept more. He has since
expanded on these studies at Caltech.

Sleep might also be a time for your brain to do a
little housekeeping. As you learn and absorb information throughout
the day, you’re constantly generating new synapses, the
junctions between neurons through which brain signals travel. But
your skull has limited space, so bedtime might be when superfluous
synapses are cleaned out.

And finally, during your daily slumber, your
brain might be replaying the events of the day, reinforcing memory
and learning. Thanos Siapas, associate professor of computation and
neural systems, is one of several scientists who have done
experiments that suggest this explanation for sleep. He and his
colleagues looked at the brain activity of rats while the rodents
ran through a maze and then again while they slept. The patterns
were similar, suggesting the rats were reliving their day while
asleep.

Of course, the real reason for sleep could be any
combination of these four ideas, Prober says. Or perhaps only one
of these hypotheses might have been true in the evolutionary past,
but as organisms evolved, they developed additional uses for
sleep.

Researchers in Prober’s lab look for the
genetic and neural systems that affect zebrafish sleeping patterns
by tweaking their genes and watching them doze off. An overhead
camera records hundreds of tiny zebrafish larvae as they swim in an
array of shallow square dishes. A computer automatically determines
whether the fish are awake or not based on whether they’re
moving or still, and whether they respond to various stimuli.
Prober has identified about 500 drugs that affect their sleeping
patterns, and now his lab is searching for the relevant genetic
pathways. By studying the fish, the researchers hope to better
understand sleep in more complex organisms like humans. “Even
if we find only a few new genes, that’ll really open up the
field,” he says. The future is promising, he adds, and for
that, it’ll be well worth staying awake.